As computer processing units (CPU's) become more powerful, compact and dense, they also run hotter, and require more efficient cooling. Air cooling is not sufficient for the newer generations of CPU's, and forced liquid cooling has been proposed, and is already being used in some cases, as an alternative. In one simple design of a so called cooling block, a conductive metal cold plate has a smooth undersurface, and a generally finned or otherwise surface area enhanced upper surface. The CPU is thermally joined to the cold plate smooth undersurface, by a compliant conductive paste, and a simple box shaped manifold is sealed over the upper surface of the cold plate. Liquid coolant is pumped into and out of the box manifold, and flows freely over and around the fins or other surface area enhancements of the cold plate to extract heat. No particular provision is made to guide or control the liquid flow over the cold plate upper surface.
Another variation in liquid cooling uses a metal cold plate (or cold sheet) which is extremely thin, and in which the fins or micro channels cut into its upper surface are also extremely fine, created by chemical etching or laser cutting. Instead of simply attaching a box shaped manifold above and over the finned upper surface, the liquid flow is guided and forced directly down into and through the fins-channels in a branched, hierarchical flow. This is achieved by sealing a manifold with at least one tier of inlet and outlet channels tightly down over and perpendicularly crossing the micro fin-channel pattern, so that flow is forced into an even and regularized checkerboard pattern. This forced and very finely distributed liquid flow is more effective and efficient in heat extraction. In order for the regularized liquid flow to occur, however, the sealing of the manifold down over the top edges of the fin pattern must be tight and complete, to prevent and flow from short circuiting or by passing the channels, which would disrupt the otherwise forced regularity of the flow pattern.
A detailed disclosure of the micro channel, checkerboard flow design may be found in the now expired U.S. Pat. No. 5,388,635 to Gruber et al. As disclosed there, a micro channeled metal cold sheet has a single or multi tiered rubber or plastic manifold vulcanized to its upper surface, and one or more CPU's thermally bonded to its lower surface. As a practical matter, it would likely be very difficult to successfully vulcanize a compliant, rubber manifold over the extremely fine channels involved without clogging at least some of the channels. Using a hard, non compliant material for the manifold instead, one unlikely to protrude into or clog any of the channels would present its own problems, however. The multiple crossing interfaces between the bottom edges of the manifold channels and top the edges of the cold sheet micro channel-fins would have to be held to very close tolerances, so that any gap would be minimized, and whatever sealing material (solder, braze cladding, adhesive, etc) that was used to fill and seal even a small gap would also be liable to run into and clog the very small micro channels.